Various derivatives of pentaerythritol phosphate are known as flame retardant additives for polymers such as polypropylene. A recent example is provided by U.S. Pat. No. 4,801,625 to W. J. Parr et al. which describes ether, ester and carbonate derivatives of pentacrythritol phosphate. The carbonate version of such compounds can be advantageously prepared by the reaction of pentaerythritol phosphate alcohol with a dihydrocarbyl carbonate as described in U.S. Pat. No. 5,235,085.